Absorbent structure with superabsorbent material

ABSTRACT

An absorbent structure made at least in part from a superabsorbent material having a retention capacity (CRC) as determined by a Centrifuge Retention Capacity Test of at least about 25 g/g and a free swell gel bed permeability (GBP) as determined by a Free Swell Gel Bed Permeability Test of at least 575×10 −9  cm 2 . In another embodiment, the absorbent structure is made at least in part from a superabsorbent material having a retention capacity (CRC) as determined by a Centrifuge Retention Capacity Test of at least about 25 g/g, an absorbency under load (AUL) at 0.9 psi as determined by an Absorbency Under Load Test of at least 18 and a free swell gel bed permeability (GBP) as determined by a Free Swell Gel Bed Permeability Test of at least about 350×10 −9  cm 2 .

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/423,709, filed Apr. 25, 2003. The entire text of the above-referencedapplication is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates generally to absorbent structures used indisposable articles such as diapers, children's training pants, femininecare articles, incontinence articles, bandages, surgical gowns,absorbent wipes and the like, and more particularly to such absorbentstructures containing a superabsorbent material having enhanced liquidhandling characteristics such as an enhanced combination of retentioncapacity, free swell gel bed permeability and/or absorbency under loadvalue.

Conventional disposable articles typically include an absorbentstructure, also sometimes referred to as an absorbent core or absorbentcomposite, formed by air-forming, air-laying or other known formingtechnique. For example, the manufacture of such an absorbent structuremay begin by fiberizing a fibrous sheet of hydrophilic material in afiberizer or other shredding or comminuting device to form discretefibers. In addition, particles or fibers of superabsorbent material,which are water insoluble, water swellable and capable of absorbing atleast about ten times their weight in 0.9 weight percent sodium chloridesolution in distilled water (saline solution), are mixed with thediscrete fibers. The hydrophilic fibers and superabsorbent material arethen entrained in an air stream and directed to a foraminous formingsurface upon which the fibers and superabsorbent material are depositedand accumulated to form the absorbent structure.

There is a continuing effort by absorbent structure manufacturers toimprove the liquid intake performance of absorbent structures to therebyreduce the tendency of such a structure to leak as it becomesincreasingly saturated during use, particularly where the structure issubjected to repeated liquid insults before being discarded. Forexample, one means of reducing the leakage of absorbent structures hasbeen the extensive use of superabsorbent materials. In addition toincreasing the amount of superabsorbent mass, recent efforts incommercial absorbent structure design have generally focused on using ahigher concentration of superabsorbent material and less fiber to makethe absorbent structure thinner and denser.

However, notwithstanding the increase in total absorbent capacityobtained by increasing the concentration of superabsorbent material,such absorbent structures may still leak during use. The leakage may bein part the result of the structure having an insufficient intake rate,e.g., the rate at which a liquid insult can be taken into and entrainedwithin the structure for subsequent absorption by the superabsorbentmaterial. More particularly, the intake rate of such absorbentstructures may decrease upon repeated insults thereof due to thetendency of the superabsorbent material within the structure to swell asit absorbs and thus restrict or otherwise block the open channelsbetween superabsorbent particles, or between the particles and thehydrophilic fibers within the absorbent structure. This phenomenon isoften referred to as a form of gel-blocking and may occur as a result ofthe superabsorbent material lacking sufficient gel integrity or reachingsuch a high degree of swelling that it tends to be easily deformableunder an external pressure, such as those loads applied by a wearerduring movement or upon sitting down.

The in-use performance of an absorbent structure may therefore relyupon 1) the ability to create open channels and void volume within theabsorbent structure and 2) the ability to maintain the openness of andaccessibility to such channels and void volume upon saturation of theabsorbent structure. The ability to create the open channels may be afunction of the ability of the superabsorbent material to absorb liquidwhile the material is under pressure as well as the ability to retainliquid and not deform while under pressure. Liquid handlingcharacteristics commonly associated with such functions include theretention capacity (CRC) and the absorbency under load (AUL) value ofthe superabsorbent material. The ability to maintain openness of andaccessibility to the channels and void volume may be in large part afunction of the gel bed permeability (GBP) of the superabsorbentmaterial. A higher GBP indicates a higher ability to maintain openchannels within the absorbent structure after the superabsorbentmaterial is saturated and fully swollen.

To date, research efforts directed toward improving the liquid handlingcharacteristics of absorbent structures have generally been focused onenhancing the gel bed permeability of the superabsorbent material withinabsorbent structures. However, such an approach has come at a cost inthe form of reduced or at least a lack of enhanced retention capacity.

There is a need, therefore, for absorbent structures incorporatingsuperabsorbent materials having a high gel bed permeability, a highretention capacity and/or a high absorbency under load value.

SUMMARY OF THE INVENTION

In one embodiment, an absorbent structure of the present inventiongenerally comprises at least in part a superabsorbent material having aretention capacity (CRC) as determined by a Centrifuge RetentionCapacity Test of at least about 25 g/g and a free swell gel bedpermeability (GBP) as determined by a Free Swell Gel Bed PermeabilityTest of at least 575×10⁻⁹ cm².

In another embodiment, an absorbent structure of the present inventiongenerally comprises at least in part a superabsorbent material having aretention capacity (CRC) as determined by a Centrifuge RetentionCapacity Test of at least about 25 g/g, an absorbency under load (AUL)at 0.9 psi as determined by an Absorbency Under Load Test of at least 18and a free swell gel bed permeability (GBP) as determined by a FreeSwell Gel Bed Permeability Test of at least about 350×10⁻⁹ cm².

Other features of the invention will be in part apparent and in partpointed out hereinafter.

DEFINITIONS

Within the context of this specification, each term or phrase below willinclude the following meaning or meanings:

“Bi-component,” or “Multi-component” fibers as used herein refers tofibers formed from two (e.g., bi-component) or more components, such asa natural fiber and a polymer or two or more polymers extruded from oneor more separate extruders, joined together to form a single fiber. Thecomponents are arranged in substantially constantly positioned distinctzones across a cross-section of the multi-component fibers and extendcontinuously along at least a portion of, and more desirably the entire,length of the fiber. The configuration of the multi-component fibers maybe, for example, a sheath/core arrangement in which one polymer issurrounded by another, a side-by-side arrangement, a pie arrangement, an“islands-in-the-sea” arrangement or other suitable arrangement.Bi-component fibers are disclosed in U.S. Pat. No. 5,108,820 to Kanekoet al., U.S. Pat. No. 4,795,668 to Krueger et al., U.S. Pat. No.5,540,992 to Marcher et al. and U.S. Pat. No. 5,336,552 to Strack et al.Bi-component fibers are also taught in U.S. Pat. No. 5,382,400 to Pikeet al. and may be used to produce crimp in the fibers by using thedifferential rates of expansion and contraction of the two (or more)polymers.

“Bonded-Carded” refers to webs that are made from staple length fiberswhich are sent through a combing or carding unit, which separates orbreaks apart and aligns the fibers in the machine direction to form agenerally machine direction-oriented fibrous non-woven web. Thismaterial may be bonded together by methods that include point bonding,through air bonding, ultrasonic bonding, adhesive bonding or othersuitable bonding technique.

“Hydrophilic” describes a material or surface which is wetted by aqueousliquids in contact therewith. The degree of wetting can, in turn, bedescribed in terms of the contact angles and the surface tensions of theliquids and materials involved. Equipment and techniques suitable formeasuring the wettability of particular materials or surfaces can beprovided by a Cahn SFA-222 Surface Force Analyzer System, or asubstantially equivalent system. When measured with this system,materials or surfaces having contact angles less than 90 degrees aredesignated “wettable” or hydrophilic, and those having contact anglesgreater than 90 degrees are designated “nonwettable” or hydrophobic.

“Meltblown” refers to fibers formed by extruding a molten thermoplasticmaterial through a plurality of fine, usually circular, die capillariesas molten threads or filaments into converging high velocity heated gas(e.g., air) streams which attenuate the filaments of moltenthermoplastic material to reduce their diameters. Thereafter, themeltblown fibers are carried by the high velocity gas stream and aredeposited on a collecting surface to form a web of randomly dispersedmeltblown fibers. Such a process is disclosed, for example, in U.S. Pat.No. 3,849,241 to Butin et al, which is incorporated herein by reference.Meltblown fibers are typically microfibers which may be continuous ordiscontinuous, are generally about 0.6 denier or smaller, and aregenerally self-bonding when deposited onto a collecting surface.

“Non-woven” or “non-woven web” refers to materials or webs that areformed without the aid of a textile weaving or knitting process. Thestructure comprises individual or groups of fibers or threads which areinterlaid, but not in an identifiable manner as in a knitted fabric.Non-woven structures have been formed from many processes such as, forexample, meltblowing processes, spunbonding processes, and bonded-cardedprocesses.

“Spunbond” refers to small diameter fibers which are formed by extrudingmolten thermoplastic material as filaments from a plurality of finecapillaries of a spinneret having a circular or other configuration,with the diameter of the extruded filaments then being rapidly reducedby an air-drawing process such as that described in U.S. Pat. No.4,340,563 to Appel et al., U.S. Pat. No. 3,692,618 to Dorschner et al.,U.S. Pat. No. 3,802,817 to Matsuki et al., U.S. Pat. Nos. 3,338,992 and3,341,394 to Kinney, U.S. Pat. No. 3,502,763 to Hartmann, U.S. Pat. No.3,502,538 to Peterson, and U.S. Pat. No. 3,542,615 to Dobo et al., eachof which is incorporated herein in its entirety by reference. Spunbondfibers are generally continuous and often have average deniers of about0.3 or larger, more particularly, between about 0.6 and about 10.

“Superabsorbent” and “Superabsorbent Material” refer to awater-swellable, water-insoluble organic or inorganic polymer and/ormaterial capable, under the most favorable conditions, of absorbing atleast about 10 times its weight and, more suitably, at least about 30times its weight in an aqueous solution containing 0.9 weight percentsodium chloride solution in water. A superabsorbent polymer is acrosslinked polymer which is capable of absorbing large amounts ofaqueous liquids and body fluids, such as urine or blood, with swellingand the formation of hydrogels, and of retaining such liquids and fluidsunder pressure in accordance with the present definition of the termsuperabsorbent.

“Thermoplastic” describes a material that softens when exposed to heatand which substantially returns to a nonsoftened condition when cooledto room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of apparatus for conducting a PermeabilityTest;

FIG. 2 is a section taken in the plane of line 2-2 of FIG. 1;

FIG. 3 is a cross-section of apparatus for conducting an AbsorbencyUnder Load Test;

FIG. 4 is a section taken in the plane of line 4-4 of FIG. 3;

FIG. 5 is a top plan of apparatus for conducting a Fluid Intake FlowbackEvaluation Test;

FIG. 6 is a section taken in the plane of line 6-6 of FIG. 5;

FIG. 7 is a cross-section of apparatus for conducting a LiquidSaturation Capacity Test; and

FIG. 8 is an elevation of apparatus for conducting a Shear Modulus Test.

DETAILED DESCRIPTION

The present invention is directed generally to absorbent structureshaving enhanced liquid handling properties, and more particularly toabsorbent structures containing a superabsorbent material that has botha high liquid retention capacity and a high free swell gel bedpermeability. The present invention is also directed generally toabsorbent articles incorporating these absorbent structures. Forexample, such absorbent articles include, without limitation, femininecare pads, interlabial products, tampons, diapers, incontinence articlessuch as pads, guards, pants and undergarments, training pants, medicalgarments, bed pads, sweat absorbing pads, shoe pads, bandages, helmetliners, wipes, etc. As another example, the absorbent structure may beuseful by itself, such as in the form of a tissue, towel, napkin or thelike.

In one embodiment, the absorbent structure is a non-woven web comprisinghydrophilic fibers and superabsorbent material. Examples of suitablehydrophilic fibers include naturally occurring organic fibers composedof intrinsically wettable material, such as cellulosic fibers. Suitablesources of cellulosic fibers include: wood fibers, such as bleachedkraft softwood or hardwood, high-yield wood fibers, andChemiThermoMechanical Pulp fibers; bagasse fibers; milkweed flufffibers; wheat straw; kenaf; hemp; pineapple leaf fibers; or peat moss.Other hydrophilic fibers, such as regenerated cellulose and curledchemically stiffened cellulose fibers may also be densified to formabsorbent structures that can expand to a higher loft when wetted. Pulpfibers may also be stiffened by the use of crosslinking agents such asformaldehyde or its derivatives, glutaraldehyde, epichlorohydrin,methylolated compounds such as urea or urea derivatives, anhydrides suchas maleic anhydride, non-methylolated urea derivatives, citric acid orother polycarboxylic acids.

One example of a suitable hydrophilic fiber is available from Bowater ofCoosa River, Ala., U.S.A. as model designation CR1654 and is a bleached,highly absorbent sulfate wood pulp containing primarily soft woodfibers. Another suitable hydrophilic fiber is available from Weyerhauserof Federal Way, Wash., U.S.A. as model designation NB-416 and is ableached southern softwood pulp.

Other examples of suitable hydrophilic fibers include synthetic fiberscomposed of cellulose or cellulose derivatives, such as rayon fibers;inorganic fibers composed of an inherently wettable material, such asglass fibers; synthetic fibers made from inherently wettablethermoplastic polymers, such as particular polyester or polyamidefibers; and synthetic fibers composed of a nonwettable thermoplasticpolymer, such as polypropylene fibers, which have been hydrophilized byappropriate means. The fibers may be hydrophilized, for example, bytreatment with silica, treatment with a material that has a suitablehydrophilic moiety and is not readily removable from the fiber, or bysheathing a nonwettable, hydrophobic fiber with a hydrophilic polymerduring or after the formation of the fiber.

For the purposes of the present invention, it is contemplated thatselected blends of the various types of fibers mentioned above may alsobe employed. Moreover, the fiber selection may instead, or mayadditionally, include bi-component or bi-constituent fibers that arehydrophilic or have been treated to be hydrophilic and are used toenhance the integrity and/or softness of the absorbent structure bybonding through heat activation.

It is also contemplated that the absorbent structure may instead, or mayadditionally, comprise hydrophobic fibers without departing from thescope of this invention. In another embodiment, the absorbent structuremay comprise only a superabsorbent material, such as by being formed thestructure using conventional foaming techniques.

Suitable superabsorbent materials may be selected from natural,biodegradable, synthetic and modified natural polymers and materials. Inaddition, the superabsorbent material may comprise inorganic materials,such as silica gels, or organic compounds such as crosslinked polymers.The term “crosslinked” used in reference to the superabsorbent materialrefers to any means for effectively rendering normally water-solublematerials substantially water insoluble but swellable. Such means caninclude, for example, physical entanglement, crystalline domains,covalent bonds, ionic complexes and associations, hydrophilicassociations such as hydrogen bonding, and hydrophobic associations orVan der Waals forces.

In one embodiment, the superabsorbent material comprises a crosslinkedsuperabsorbent polymer or combination of polymers comprising a) fromabout 55 to about 99.9 weight (wt.) percent of polymerizable unsaturatedacid group containing monomers; b) from about 0.001 to about 5.0 wt.percent of internal crosslinking agent; c) from about 0.001 to about 5.0wt. percent of a surface crosslinking agent applied to the particlesurface; d) from 0 to about 5 wt. percent of a penetration modifierapplied to the surface of the particle immediately before, during orimmediately after surface crosslinking; e) from 0 to about 5 wt. percentof a multivalent metal salt on the surface; f) from about 0.01 to about5 wt. percent of an insoluble, inorganic powder; and g) from about 0 toabout 2 wt. percent surface active agent on the surface, wherein thesuperabsorbent material has a degree of neutralization of more thanabout 25 percent.

The crosslinked superabsorbent polymer is obtained by the initialpolymerization of from about 55 to about 99.9 wt. percent ofpolymerizable unsaturated acid group containing monomers. Suitablemonomers include those containing carboxyl groups, such as acrylic acid,methacrylic acid or 2-acrylamido-2-methylpropanesulfonic acid, ormixtures of these monomers are preferred here. It is preferable for atleast about 50 wt. %, and more preferably at least about 75 wt. % of theacid groups to be carboxyl groups. The acid groups are neutralized tothe extend of at least about 25 mol %, that is, the acid groups arepreferably present as sodium, potassium or ammonium salts. The degree ofneutralization is preferably at least about 50 mol %. It is preferred toobtain polymers obtained by polymerization of acrylic acid ormethacrylic acid, the carboxyl groups of which are neutralized to theextent of 50-80 mol %, in the presence of internal crosslinking agents.

Further monomers, which can be used for the preparation of thesuperabsorbent polymers, are 0-40 wt. % of ethylenically unsaturatedmonomers which can be copolymerized with a) as set forth above, such ase.g. acrylamide, methacrylamide, hydroxyethyl acrylate,dimethylaminoalkyl (meth)-acrylate, ethoxylated (meth)-acrylates,dimethylaminopropylacrylamide or acrylamidopropyltrimethylammoniumchloride. More than 40 wt. % of these monomers can impair theswellability of the polymers.

The internal crosslinking agent has at least two ethylenicallyunsaturated double bonds or one ethylenically unsaturated double bondand one functional group which is reactive towards acid groups of thepolymerizable unsaturated acid group containing monomers or severalfunctional groups which are reactive towards acid groups can be used asthe internal crosslinking component and which is present during thepolymerization of the polymerizable unsaturated acid group containingmonomers.

Examples of internal crosslinking agents include aliphatic unsaturatedamides, such as methylenebisacryl- or -methacrylamide orethylenebisacrylamide, and furthermore aliphatic esters of polyols oralkoxylated polyols with ethylenically unsaturated acids, such asdi(meth)acrylates or tri(meth)acrylates or butanediol or ethyleneglycol, polyglycols or trimethylolpropane, di- and triacrylate esters oftrimethylolpropane which is preferably oxyalkylated, preferablyethoxylated, with 1 to 30 mol of alkylene oxide, acrylate andmethacrylate esters of glycerol and pentaerythritol and of glycerol andpentaerythritol oxyethylated with preferably 1 to 30 mol of ethyleneoxide and furthermore alkyl compounds, such as alkyl(meth)acrylate,alkoxylated allyl(meth)acrylate reacted with preferably 1 to 30 mol ofethylene oxide, trially cyanurate, triallyl isocyanurate, maleic aciddiallyl ester, poly-allyl esters, tetraallyloxyethane, traillylamine,tetraallylethylenediamine, diols, polyols, hydroxyl allyl or acrylatecompounds and allyl esters of phosphoric acid or phosphorous acid, andfurthermore monomers which are capable of crosslinking, such asN-methylol compounds of unsaturated amides, such as of methacrylamide oracrylamide, and the esthers derived there from. Ionic crosslinkers suchas multivalent metal salts may also be employed. Mixtures of thecrosslinking agents mentioned can also be employed. The content of theinternal crosslinking agents is from about 0.01 to about 5 wt. %, andpreferably from about 0.1 to about 3.0 wt. %, based on the total amountof the polymerizable unsaturated acid group containing monomers.

Initiators, such as e.g. azo or peroxo compounds, redox systems or UVinitiators, (sensitizers), and/or radiation are used for initiation ofthe free-radical polymerization.

The superabsorbent polymer is surface crosslinked after polymerization.Surface crosslinking refers to any process that increases thecrosslinking density of the polymer matrix in the vicinity of thesuperabsorbent particle surface with respect to the crosslinking densityof the particle interior. The superabsorbent polymers are typicallysurface crosslinked by the addition of a surface crosslinking agent.Preferred surface crosslinking agents include chemicals with one or morefunctional groups, which are reactive towards pendant groups of thepolymer chains, typically the acid groups. The content of the surfacecrosslinking agents is from about 0.01 to about 5 wt. %, and moresuitably from about 0.1 to about 3.0 wt. %, based on the weight of thedry polymer. A heating step is preferred after addition of the surfacecrosslinking agent.

The superabsorbent polymer can be coated with an alkaline carbonatefollowed by heating to effect surface crosslinking to improve thesurface crosslinking density and the gel strength characteristics of thesuperabsorbent material. More specifically a surface crosslinking agentis coated onto the superabsorbent particle by mixing the polymer with anaqueous alcoholic solution of the alkylene carbonate surfacecrosslinking agent. The amount of alcohol is determined by thesolubility of the alkylene carbonate and is kept as low as possible fortechnical reasons, for instance protection against explosions. Suitablealcohols are methanol, ethanol, butanol, or butyl glycol as well asmixtures of these alcohols. The preferred solvent is water, whichtypically is used in an amount of 0.3 to 5.0% by weight, relative toparticulate superabsorbent polymer. In some instances, the alkylenecarbonate surface crosslinking agent is dissolved in water, without anyalcohol. It is also possible to apply the alkylene carbonate surfacecrosslinking agent from a powder mixture, for example, with an inorganiccarrier material, such as SiO₂, or in the vapor state by sublimation ofthe alkylene carbonate.

To achieve the desired surface crosslinking properties, the alkylenecarbonate is suitably distributed evenly on the particulatesuperabsorbent polymer. For this purpose, mixing is effected in suitablemixers, such as fluidized bed mixers, paddle mixers, milling rolls, ortwin-worm mixers. It is also possible to carry out the coating of theparticulate superabsorbent polymer during one of the process steps inthe production of the particulate superabsorbent polymer. A particularlysuitable process for this purpose is the inverse suspensionpolymerization process.

The thermal treatment, which follows the coating treatment, is carriedout as follows. In general, the thermal treatment is at a temperaturebetween 100 and 300° C. However, if the alkylene carbonates are used,then the thermal treatment is suitably at a temperature between 150 and250° C. The treatment temperature depends on the dwell time and the kindof alkylene carbonate. At a temperature of 150° C., the thermaltreatment is carried out for one hour or longer. On the other hand, at atemperature of 250° C., a few minutes, e.g., 0.5 to 5 minutes, aresufficient to achieve the desired surface crosslinking properties. Thethermal treatment may be carried out in conventional dryers or ovens.

While particles are used by way of example as the physical form of thesuperabsorbent polymer, the invention is not limited to this form and isapplicable to other forms such as fibers, foams, films, beads, rods andthe like.

The superabsorbent polymer can comprise from 0 to about 5 wt. % of apenetration modifier that is added immediately before, during orimmediately after the surface crosslinking agent. Examples ofpenetration modifiers include compounds which alter the penetrationdepth of surface-modifying agents in to the superabsorbent polymerparticle, fiber, film, foam or bead by changing the viscosity, surfacetension, ionic character or adhesion of said agents or medium in whichthese agents are implied. Preferred penetration modifiers arepolyethylene glycols, tetraethylene glycol dimethyl ether, monovalentmetal salts, surfactants and water soluble polymers.

The superabsorbent polymer according to the invention can comprise from0 to about 5 wt. % of a multivalent metal salt, based on the weight ofthe mixture, on the surface of the polymer. The multivalent metal saltis preferably water soluble. Examples of preferred metal cations includecations of Al, Fe, Zr, Mg and Zn. Preferably, the metal cation has avalence of at least +3, with Al being most suitable. Examples ofsuitable anions in the multivalent metal salt include halides,chlorohydrates, sulfates, nitrates and acetates, with chlorides,sulfates, chlorohydrates and acetates being preferred, chlorohydratesand sulfates being more preferred and sulfates being the most preferred.Aluminum sulfate is the most suitable multivalent metal salt and isreadily commercially available. A suitable form of aluminum sulfate ishydrate aluminum sulfate, preferably aluminum sulfate having from 12 to14 waters of hydration. It is understood that mixtures of multivalentmetal salts can be employed and remain within the scope of thisinvention.

The polymer and multivalent metal salt are suitably mixed by dryblending, or more suitably by being blended in solution, and mostsuitably an aqueous solution, using means well known to those skilled inthe art. With dry blending, a binder may be employed in an amountsufficient to ensure that a substantially uniform mixture of the saltand the polymer is maintained. The binder may be water or a nonvolatileorganic compound having a boiling point of at least 150° C. Examples ofsuitable binders include water, polyols such a propylene glycol,glycerin and poly(ethylene glycol).

The superabsorbent polymer can also comprise from about 0.01 to about 5wt. % of water-insoluble, inorganic powder. Examples of suitablewater-insoluble, inorganic powders include silicon dioxide, silicicacid, silicates, titanium dioxide, aluminum oxide, magnesium oxide, zincoxide, talc, calcium phosphate, clays, diatomataceous earth, zeolites,bentonite, kaolin, hydratalcite, activated clays, etc. The insolubleinorganic powder additive may be a single compound or a mixture ofcompounds selected from the above list. Of all of these examples,microscopic noncrystal silicon dioxide or aluminum oxide are mostsuitable. Further, a suitable particle diameter of the inorganic powderis 1,000 μm or smaller, and more suitably 100 μm or smaller.

The superabsorbent polymer may also include the addition of from 0 toabout 5 wt. % of a surfactant to the polymer particle surface. It ispreferred that these be added immediately prior to, during orimmediately after the surface crosslinking step. Examples of suchsurfactants include anionic, non-ionic, cationic and amphoteric surfaceactive agents, such as fatty acid salts, dialykl sulfo-succinate, alkylphosphate salt, and polyoxyethylene alkyl sulfate sale; polyoxyethylenealkyl ether, polyoxyethylene alkyl phenol ether, polyoxyethylene fattyacid ester, sorbitan fatty acid ester, polyoxy sorbitan fatty acidester, polyoxyethylene alkylamine, fatty acid esters, andoxyethylene-oxypropylene block polymer; alkyl amine salts, quaternaryammonium salts; and lauryl dimethylamine oxide. However, it is notnecessary to restrict the surfactant to those mentioned above. Suchsurfactants may be used individually, or in combination.

The superabsorbent polymer may also include from 0 to about 30 wt. % ofwater-soluble polymer, such as partly or completely hydrolyzedpolyvinyle acetate, polyvinylpyrrolidone, starch or starch derivatives,polyglycols or polyacrylic acids, preferably in polymerized-in form. Themolecular weight of these polymers is not critical as long as they arewater-soluble. Preferred water-soluble polymers are starch and polyvinylalcohol. The preferred content of such water-soluble polymers in theabsorbent polymer according to the invention is 0-30 wt. %, preferably0-5 wt. %, based on the total amount of components a) to d). Thewater-soluble polymers, preferably synthetic polymers, such as polyvinylalcohol, can also serve as a graft base for the monomers to bepolymerized.

It is sometimes desirable to employ surface additives that performseveral roles during surface modifications. For example, a singleadditive may be a surfactant, viscosity modifier and react to crosslinkpolymer chains.

The superabsorbent polymer may also include from 0 to about 2.0 wt. % ofdedusting agents, such as hydrophilic and hydrophobic dedusting agentssuch as those described in U.S. Pat. Nos. 6,090,875 and 5,994,440.

Further additives may optionally be employed, e.g., odor-bindingsubstances such as cyclodextrins, zeolites, inorganic or organic saltsand similar materials; anti-caking additives, flow modification agentsand the like.

The superabsorbent polymer is suitably prepared by two methods. Thepolymer can be prepared continuously or discontinuously in a large-scaleindustrial manner by the above-mentioned known process, theafter-crosslinking being carried out accordingly.

According to the first method, the partly neutralized monomer, suitablyacrylic acid, is converted into a gel by free-radical polymerization inaqueous solution in the presence of crosslinking agents and optionallyfurther components, and the gel is comminuted, dried, ground and sievedoff to the desired particle size. This solution polymerization can becarried out continuously or discontinuously.

Inverse suspension and emulsion polymerization can also be used forpreparation of the superabsorbent polymer. According to these processes,an aqueous, partly neutralized solution of monomers, preferably acrylicacid, is dispersed in a hydrophobic, organic solvent with the aid ofprotective colloids and/or emulsifiers and the polymerization is startedby free radical initiators. The internal crosslinking agents are eitherdissolved in the monomer solution and metered in together with this, orare added separately and optionally during the polymerization. Theaddition of a water-soluble polymer as the graft base optionally takesplace via the monomer solution or by direct introduction into the oilyphase. The water is then removed azeotropically from the mixture and thepolymer is filtered off and optionally dried. Internal crosslinking canbe carried out by polymerizing in a polyfunctional crosslinking agentdissolved in the monomer solution and/or by reaction of suitablecrosslinking agents with functional groups of the polymer during thepolymerization steps.

In one particular embodiment, the superabsorbent material suitable foruse in the absorbent structure of the present invention comprises acrosslinked superabsorbent polymer comprising either at least about 75weight percent anionic polymer/polymers or at least about 75 weightpercent cationic polymer/polymers. More suitably, the superabsorbentmaterial comprises a crosslinked polymer comprising either at leastabout 85 weight percent anionic polymer/polymers or at least about 85weight percent cationic polymer/polymers, and even more suitably eitherat least about 90 weight percent anionic polymer/polymers or at leastabout 90 weight percent cationic polymer/polymers.

An anionic polymer is intended to refer to a polymer comprising afunctional group or groups capable of becoming negatively charged ionsupon ionization in an aqueous solution. In general, suitable functionalgroups for an anionic polymer include, but are not limited to, carboxylgroups, sulfonate groups, sulphate groups, sulfite groups, and phosphategroups. Suitably, the functional groups are carboxyl groups. It ispreferred that these functional groups are in neutralized form. Asuitable degree of neutralization is at least 50%, more suitably atleast 60%, and even more suitably at least 70%.

A cationic polymer is intended to refer to a polymer comprising afunctional group or groups capable of becoming positively charged ionsupon ionization in an aqueous solution. In general, suitable functionalgroups for a cationic polymer include, but are not limited to, primary,secondary, or tertiary amino groups, imino groups, imido groups, amidogroups, and quaternary ammonium groups. It is suitable that thesefunctional groups are in neutralized form. A suitable degree ofneutralization is at least 50%, more suitably at least 60%, and evenmore suitably at least 70%.

Examples of synthetic anionic superabsorbent polymers include the alkalimetal and ammonium salts or partial salts of poly(acrylic acid),poly(methacrylic acid), hydrolyzed poly(acrylamides), maleic anhydridecopolymers with vinyl ethers and alpha-olefins, poly(vinyl acetic acid),poly(vinyl sulfonic acid), poly(vinyl phosphonic acid), poly(vinylethers), poly(vinyl pyrrolidone), poly(vinylmorpholinone), poly(vinylalcohol), and mixtures and copolymers thereof. Examples of natural basedanionic polymers include the salts or partial salts of carboxymethylcellulose, carboxymethyl starch, alginates, and carrageenans. Also,synthetic polypeptides such as polyaspartic acid and polyglutamic acidcan be examples of the anionic polymers. Examples of synthetic cationicsuperabsorbent polymers include the salts or partial salts of poly(vinylamines), poly(allylamines), poly(ethylene imine), poly(amino proanolvinyl ethers), poly(acrylamidopropyl trimethyl ammonium chloride),poly(diallyldimethyl ammonium chloride). Examples of natural basedcationic polymers include partially deacetalated chitin, chitosan andchitosan salts. Also synthetic polypeptides such as polyasparagins,polylysines, polyglutamines, polyarginines can be examples of thecationic polymers.

In one embodiment, the superabsorbent material used in making theabsorbent structure is in the form of discrete particles. Superabsorbentmaterial particles can be of any suitable shape, for example, spiral orsemi-spiral, cubic, rod-like, polyhedral, etc. Particle shapes having alarge greatest dimension/smallest dimension ratio, like needles, flakes,and fibers, are also contemplated for use herein. Conglomerates ofparticles of superabsorbent material may also be used in the absorbentstructure. The superabsorbent materials may be of various length andcross-sectional dimensions.

In accordance with the present invention, the superabsorbent materialshave certain liquid handling characteristics, including a suitable freeswell gel bed permeability (GBP), a suitable absorbency under load value(AUL), a suitable centrifuge retention capacity (CRC), a suitableabsorption against pressure (AAP) value and a suitable shear modulus(G′), all of which are measurable using the following tests.

Free Swell Gel Bed Permeability Test

As used herein, the Free Swell Gel Bed Permeability (GBP) Testdetermines the permeability of a swollen bed of superabsorbent materialunder what is commonly referred to as “free swell” conditions. The term“free swell” means that the superabsorbent material is allowed to swellwithout a swell restraining load upon absorbing test solution as will bedescribed. A suitable apparatus for conducting the Free Swell Gel BedPermeability Test is shown in FIGS. 1 and 2 and indicated generally at28. The test apparatus 28 comprises a sample container, generallyindicated at 30, and a piston, generally indicated at 36. The piston 36comprises a cylindrical LEXAN shaft 38 having a concentric cylindricalhole 40 bored down the longitudinal axis of the shaft. Both ends of theshaft 38 are machined to provide upper and lower ends respectivelydesignated 42, 46. A weight, indicated as 48, rests on one end 42 andhas a cylindrical hole 48 a bored through at least a portion of itscenter.

A circular piston head 50 is positioned on the other end 46 and isprovided with a concentric inner ring of seven holes 60, each having adiameter of about 0.95 cm, and a concentric outer ring of fourteen holes54, also each having a diameter of about 0.95 cm. The holes 54, 60 arebored from the top to the bottom of the piston head 50. The piston head50 also has a cylindrical hole 62 bored in the center thereof to receiveend 46 of the shaft 38. The bottom of the piston head 50 may also becovered with a biaxially stretched 400 mesh stainless steel screen 64.

The sample container 30 comprises a cylinder 34 and a 100 mesh stainlesssteel cloth screen 66 that is biaxially stretched to tautness andattached to the lower end of the cylinder. A superabsorbent materialsample, indicated as 68 in FIG. 1, is supported on the screen 66 withinthe cylinder 34 during testing.

The cylinder 34 may be bored from a transparent LEXAN rod or equivalentmaterial, or it may be cut from a LEXAN tubing or equivalent material,and has an inner diameter of about 6 cm (e.g., a cross-sectional area ofabout 28.27 cm²), a wall thickness of about 0.5 cm and a height ofapproximately 5 cm. Drainage holes (not shown) are formed in thesidewall of the cylinder 34 at a height of approximately 4.0 cm abovethe screen 66 to allow liquid to drain from the cylinder to therebymaintain a fluid level in the sample container at approximately 4.0 cmabove the screen 66. The piston head 50 is machined from a LEXAN rod orequivalent material and has a height of approximately 16 mm and adiameter sized such that it fits within the cylinder 34 with minimumwall clearance but still slides freely. The shaft 38 is machined from aLEXAN rod or equivalent material and has an outer diameter of about 2.22cm and an inner diameter of about 0.64 cm.

The shaft upper end 42 is approximately 2.54 cm long and approximately1.58 cm in diameter, forming an annular shoulder 47 to support theweight 48. The annular weight 48 has an inner diameter of about 1.59 cmso that it slips onto the upper end 42 of the shaft 38 and rests on theannular shoulder 47 formed thereon. The annular weight 48 can be madefrom stainless steel or from other suitable materials resistant tocorrosion in the presence of the test solution, which is 0.9 wt. %sodium chloride solution in distilled water. The combined weight of thepiston 36 and annular weight 48 equals approximately 596 grams (g),which corresponds to a pressure applied to the sample 68 of about 0.3pounds per square inch (psi), or about 20.7 dynes/cm², over a samplearea of about 28.27 cm².

When the test solution flows through the test apparatus during testingas described below, the sample container 30 generally rests on a 16 meshrigid stainless steel support screen (not shown). Alternatively, thesample container 30 may rest on a support ring (not shown) diametricallysized substantially the same as the cylinder 34 so that the support ringdoes not restrict flow from the bottom of the container.

To conduct the Gel Bed Permeability Test under “free swell” conditions,the piston 36, with the weight 48 seated thereon, is placed in an emptysample container 30 and the height from the bottom of the weight 48 tothe top of the cylinder 34 is measured using a caliper or suitable gaugeaccurate to 0.01 mm. It is important to measure the height of eachsample container 30 empty and to keep track of which piston 36 andweight 48 is used when using multiple test apparatus. The same piston 36and weight 48 should be used for measurement when the sample 68 is laterswollen following saturation.

The sample to be tested is prepared from superabsorbent materialparticles which are prescreened through a U.S. standard 30 mesh screenand retained on a U.S. standard 50 mesh screen. As a result, the testsample comprises particles sized in the range of about 300 to about 600microns. The particles can be prescreened by hand or automatically.Approximately 0.9 grams of the sample is placed in the sample container30, and the container, without the piston 36 and weight 48 therein, isthen submerged in the test solution for a time period of about 60minutes to saturate the sample and allow the sample to swell free of anyrestraining load.

At the end of this period, the piston 36 and weight 48 assembly isplaced on the saturated sample 68 in the sample container 30 and thenthe sample container 30, piston 36, weight 48, and sample 68 are removedfrom the solution. The thickness of the saturated sample 68 isdetermined by again measuring the height from the bottom of the weight48 to the top of the cylinder 34, using the same caliper or gauge usedpreviously provided that the zero point is unchanged from the initialheight measurement. The height measurement obtained from measuring theempty sample container 30, piston 36, and weight 48 is subtracted fromthe height measurement obtained after saturating the sample 68. Theresulting value is the thickness, or height “H” of the swollen sample.

The permeability measurement is initiated by delivering a flow of thetest solution into the sample container 30 with the saturated sample 68,piston 36, and weight 48 inside. The flow rate of test solution into thecontainer is adjusted to maintain a fluid height of about 4.0 cm abovethe bottom of the sample container. The quantity of solution passingthrough the sample 68 versus time is measured gravimetrically. Datapoints are collected every second for at least twenty seconds once thefluid level has been stabilized to and maintained at about 4.0 cm inheight. The flow rate Q through the swollen sample 68 is determined inunits of grams/second (g/s) by a linear least-square fit of fluidpassing through the sample 68 (in grams) versus time (in seconds).

Permeability in cm² is obtained by the following equation:K=[Q*H*Mu]/[A*Rho*P]

-   -   where K=Permeability (cm²), Q=flow rate (g/sec), H=height of        sample (cm), Mu liquid viscosity (poise) (approximately one        centipoises for the test solution used with this Test),        A=cross-sectional area for liquid flow (cm²), Rho=liquid density        (g/cm³) (approximately one g/cm³, for the test solution used        with this Test) and P=hydrostatic pressure (dynes/cm²) (normally        approximately 3,923 dynes/cm²). The hydrostatic pressure is        calculated from        P=Rho*g*h    -   where Rho=liquid density (g/cm³), g=gravitational acceleration,        nominally 981 cm/sec², and h=fluid height, e.g., 4.0 cm for the        Free Swell Gel Bed Permeability Test described herein.

A minimum of three samples is tested and the results are averaged todetermine the free swell gel bed permeability of the sample. The samplesare tested at 23±1 degrees Celcius at 50±2 percent relative humidity.

Absorbency Under Load Test

The Absorbency Under Load (AUL) Test measures the ability of thesuperabsorbent material to absorb a 0.9 weight percent solution ofsodium chloride in distilled water at room temperature (test solution)while the material is under a 0.9 psi load. Apparatus 106 for conductingthe AUL Test is shown in FIG. 3 and comprises a Demand Absorbency Tester(DAT), generally indicated at 100, which is similar to the GravimetricAbsorbency Test System (GATS) available from M/K Systems of Danners,Mass., U.S.A., and to the system described by Lichstein at pages 129-142of the INDA Technological Symposium Proceedings, March 1974.

The test apparatus further comprises a test stand, generally indicatedat 101 (FIG. 4) having a cavity 102 formed therein and a porous plate103 seated in the cavity and having a central porous area of about 2.54cm diameter formed by a plurality of bores 104 extending through theplate. The cavity 102 shown in FIG. 4 has a diameter of about 3.2 cm andthe porous plate 103 has a diameter of about 3.1 cm and comprises sevenbores 104, each having a diameter of about 0.3 cm. One of the bores 104is centrally located and the remaining six bores are concentricallypositioned about the central bore with the spacing from the center ofthe central bore to the center of each adjacent bore is about onecentimeter.

A sample container for containing a sample 110 of superabsorbentmaterial to be tested comprises a cylinder 112 and a stainless steelcloth screen 114 that is biaxially stretched to tautness and attached tothe lower end of the cylinder. The cylinder 112 may be bored from atransparent LEXAN rod or equivalent material, or it may be cut from aLEXAN tubing or equivalent material, and has an inner diameter of aboutone inch (about 2.54 cm). The stainless steel cloth screen 114 issuitably a 100 mesh screen.

A disc, or piston 116 is machined from a LEXAN rod, Plexiglas orequivalent material and has a diameter sized such that it fits withinthe cylinder 112 with minimum wall clearance but still slides freely.The height of the piston 116 is approximately 0.8 cm and the weight ofthe piston is suitably about 4.4 grams to provide a load over thecross-sectional area of the sample in the container of about 0.01 psi. Aweight 118 is sized (e.g., having a diameter of about 2.5 cm) forseating on the piston 116 to increase the load (e.g., in addition to theweight of the piston) on the sample. For example, a weight of about 317grams is used to provide a load (e.g., including the piston weight) ofabout 0.9 psi over the cross-sectional area of the sample in thecontainer.

The cavity 102, and hence the porous plate 103, is in fluidcommunication with a reservoir 120 containing test solution (0.9 weightpercent sodium chloride solution in distilled water at room temperature)via a suitable conduit 122. As shown in FIG. 3, the reservoir 120 isseated on an electrostatic balance 108.

A sample 110 of superabsorbent material weighing about 0.160 grams isprepared by screening the particles through a U.S. standard 30 meshscreen and retaining the particles on a U.S. standard 50 mesh screen sothat the sample comprises particles in the size range of about 300 toabout 600 microns. The sample is weighed on suitable weighing paper andthen loaded into the sample container (with the piston 116 removed) sothat the particles overlay the screen at the bottom of the container.The sample container is gently tapped to level the bed of particles inthe container.

The AUL Test is initiated by placing a circular piece of GF/A glassfilter paper 124 onto the porous plate 103 over the bores 104 formedtherein and allowed to become saturated by test solution delivered fromthe reservoir 120 to the porous plate via the conduit 122. The paper 124is suitably sized larger than the inner diameter of the cylinder 112 andsmaller than the outer diameter thereof to ensure good contact whileinhibiting evaporation over the bores 104. The electrostatic balance 108is zeroed at this time. The piston 116 and weight 118 are placed on thesample within the container and the container (with the sample, pistonand weight therein) is placed on the plate 103 over the saturated glassfilter paper 124 to allow test solution to be taken into the sample inthe container via the conduit 122, bores 104 in the plate 102 and thefilter paper.

The electrostatic balance 108 is used to measure the flow of testsolution to the sample over a period of about 60 minutes. The amount (ingrams) of solution taken into the sample after about 60 minutes dividedby the dry weight of the sample (e.g., about 0.160 grams) is the AULvalue of the sample in grams of liquid per gram weight of sample.

Two checks can be made to ensure the accuracy of the measurement. First,the height the piston 116 rises above the screen 114 at the bottom ofthe sample container multiplied by the cross-sectional area of thepiston should roughly equal the amount of solution picked up by thesample over the 60 minute period. Second, the sample container can beweighed before (e.g., while the superabsorbent material is dry) andafter the test and the difference in weight should roughly equal theamount of solution picked up by the sample over the 60 minute period.

A minimum of three tests is performed and the results are averaged todetermine the AUL value at 0.9 psi. The samples are tested at 23±1degrees Celcius at 50±2 percent relative humidity.

Centrifuge Retention Capacity Test

The Centrifuge Retention Capacity (CRC) Test measures the ability of thesuperabsorbent material to retain liquid therein after being saturatedand subjected to centrifugation under controlled conditions. Theresultant retention capacity is stated as grams of liquid retained pergram weight of the sample (g/g). The sample to be tested is preparedfrom particles which are prescreened through a U.S. standard 30 meshscreen and retained on a U.S. standard 50 mesh screen. As a result, thesample comprises particles sized in the range of about 300 to about 600microns. The particles can be prescreened by hand or automatically andare stored in a sealed airtight container until testing.

The retention capacity is measured by placing 0.2±0.005 grams of theprescreened sample into a water-permeable bag which will contain thesample while allowing a test solution (0.9 weight percent sodiumchloride in distilled water) to be freely absorbed by the sample. Aheat-sealable tea bag material, such as that available from DexterCorporation of Windsor Locks, Conn., U.S.A., as model designation 1234Theatsealable filter paper works well for most applications. The bag isformed by folding a 5-inch by 3-inch sample of the bag material in halfand heat-sealing two of the open edges to form a 2.5-inch by 3-inchrectangular pouch. The heat seals should be about 0.25 inches inside theedge of the material. After the sample is placed in the pouch, theremaining open edge of the pouch is also heat-sealed. Empty bags arealso made to serve as controls. Three samples (e.g., filled and sealedbags) are prepared for the test. The filled bags must be tested withinthree minutes of preparation unless immediately placed in a sealedcontainer, in which case the filled bags must be tested within thirtyminutes of preparation.

The bags are placed between two TEFLON® coated fiberglass screens having3 inch openings (Taconic Plastics, Inc., Petersburg, N.Y.) and submergedin a pan of the test solution at 23 degrees Celsius, making sure thatthe screens are held down until the bags are completely wetted. Afterwetting, the samples remain in the solution for about 30±1 minutes, atwhich time they are removed from the solution and temporarily laid on anon-absorbent flat surface. For multiple tests, the pan should beemptied and refilled with fresh test solution after 24 bags have beensaturated in the pan.

The wet bags are then placed into the basket of a suitable centrifugecapable of subjecting the samples to a g-force of about 350. Onesuitable centrifuge is a Heraeus LaboFuge 400 having a water collectionbasket, a digital rpm gauge, and a machined drainage basket adapted tohold and drain the bag samples. Where multiple samples are centrifuged,the samples must be placed in opposing positions within the centrifugeto balance the basket when spinning. The bags (including the wet, emptybags) are centrifuged at about 1,600 rpm (e.g., to achieve a targetg-force of about 350), for 3 minutes. The bags are removed and weighed,with the empty bags (controls) being weighed first, followed by the bagscontaining the samples. The amount of solution retained by the sample,taking into account the solution retained by the bag itself, is thecentrifuge retention capacity (CRC) of the sample, expressed as grams offluid per gram of sample. More particularly, the retention capacity isdetermined as: $\frac{\begin{matrix}{{{sample}\text{/}{bag}\quad{weight}\quad{after}\quad{centrifuge}} -} \\{{{empty}\quad{bag}\quad{weight}\quad{after}\quad{cetrifuge}} - {{dry}\quad{sample}\quad{weight}}}\end{matrix}}{{dry}\quad{sample}\quad{weight}}$

The three samples are tested and the results are averaged to determinethe retention capacity (CRC) of the superabsorbent material. The samplesare tested at 23±1 degrees Celcius at 50±2 percent relative humidity.

Absorption Against Pressure (AAP) Test

The ability of a water-absorbing polymerizate to absorb liquid from areservoir under a defined pressure (Absorption Against Pressure (AAP)(0.7 psi=49 g/cm²)) is determined as follows: a 900 mg sample of thesuperabsorbent material is weighed in a plastic cylinder (innerdiameter=6 cm, height=5 cm) having a screen fabric (mesh width=400 mesh)as bottom, dispersed uniformly, and weighted using a defined weight inthe form of a plastic plate (diameter=5.98 cm), together with a metalpiston (diameter=5.98 cm). The plastic plate is situated between thesample and the metal piston. Thereafter, the entire testing unit isplaced on a glass filter plate (diameter=12 cm, porosity=0) which iscovered with a filter paper and soaked with a 0.9% NaCl solution. Thefilter plate is embedded in the NaCl solution up to its top edge. Thesample is allowed to absorb liquid for 60 minutes:

The plastic spacer and then the stainless steel weight are carefullyplaced into the cylinder. The weight of the completed AAP apparatus isrecorded (A). The stainless steel weight is sized to exert a pressureload of about 49 g/cm². (It is noted that 49 g/cm²=0.7 psi).

After 1 hour, the apparatus with the swollen sample is re-weighed, andthe weight recorded (B). The gram amount of the NaCl solution that hadbeen retained per gram of sample is calculated according to thefollowing equation:AAP=(B−A)/E

-   -   where AAP is in g/g at 0.7 psi. A is the weight in g of the AAP        apparatus with the sample prior to absorbing the test solution.        B is the weight in g of the AAP apparatus with the sample after        absorbing the test solution for 1 hour and E is the dry weight        in g of the sample.

Shear Modulus Test

The Shear Modulus Test measures the gel strength, or gel deformationtendency, of the superabsorbent material. The shear modulus is measuredby a procedure that involves the use of a Rank Brothers PulseShearometer (FIG. 8) to measure the velocity of a torsional shear waverthrough the swollen superabsorbent material. This method avoids many ofthe problems associated with measuring the shear modulus of surfacecrosslinked superabsorbents using a traditional constant stress orconstant strain rheometer or rheometers that rely on measuring the phaseangle shift between stress and strain. The Shearometer, indicatedgenerally in FIG. 8 at 170, comprises a circular lower plate, or disk172 onto which a swollen sample of the superabsorbent material isplaced. For this Test, reference is made to the operating manual “TheSimple Solution to Shear Modulus Measurements” for the Rank PulseShearometer™. The instrument is constructed in such a way that atorsional shear wave can be propagated between a pair of parallel disks172 and 174. Each disc is mounted on a piezoelectric transducer: onebeing used to initiate the shear wave, the other to detect the arrivalof this wave a short time later. The separation of the disks can bevaried by means of a screw adjustment 176 and then measured with a dialgauge 178. The propagation time of the shear wave is measured for eachgiven disk separation. It is then possible to determine the wavevelocity from the slope of a graph of propagation time plotted againstdisk separation. A value of shear modulus can then be calculated fromthe following approximation:G=ρV ²

-   -   wherein G is the shear modulus in NM⁻²; ρ is the density of the        sample in kg.m⁻³ and V is the wave propagation velocity in ms⁻¹.

The sample being tested is swollen to its gel volume in a syntheticurine. Excess free synthetic urine is removed from the sample byblotting on two paper towels for exactly one minute, strain.

The shear modulus (G′) of the superabsorbent sample is calculated fromthe following formula:G′=Density×(shear wave velocity)×(shear wave velocity).

The elasticity of the sample may be related to the velocity of the wavein the following manner: For a passage of a shear wave through thesample, the storage component of the dynamic modulus (the elasticity),G′, can be represented by the following equation:G′=[V ²ρ(1−n ²)]/(1+n ²)²

-   -   wherein V is the propagation velocity of light; ρ is the density        of the sample; and n is the ratio of the wavelength to the        critical damping length. Measurements of the shear modulus can        be obtained through consultancy groups such as the Bristol        Colloid Center, University of Bristol, Bristol UK. In addition        Rank Shearometers are offered on the Internet.

Preparation for performing the shear modulus test includes preparingsynthetic urine which is made of 1% aqueous Triton X-100, 7.50 g; sodiumchloride 30.00 g; anhydrous CaCl₂, 0.68 g; MgCl₂6H₂O 1.80 g; and DIwater 3000.0 g.

About 90 g of synthetic urine are placed into 3 large beakers. Then anapproximately 3.00 g sample of superabsorbent material is placed intoeach of three aluminum weighing pans. Each sample is added to arespective beaker of synthetic urine and timing begins. Each sample isallowed to swell to its equilibrium value, typically for 30 minutes.Each sample is stirred to ensure uniform liquid distribution. A largemetal spatula is used to remove the hydrated samples from the beakersand spread evenly on 2 Wipe Alls L20 Kimtowels®, available fromKimberly-Clark, which are folded in half and stacked. The samples areblotted for exactly 60 seconds on the Wipe Alls. The spatula is used tospread the samples out over the paper toweling, only lightly pressingthe samples onto the towel. No more force is applied than that requiredto distribute the sample. The sample is scraped up with the spatula andreturned to the beakers after 60 seconds. The beakers are covered withfoil or film until the samples are measured.

The shear moduli of the samples are measured within one hour of samplepreparation. Each sample is transferred to a shearometer tube and placedon the lower disk 172, filling the shearometer tube to a height of atleast 18 mm above the lower disk. The top disk 174 is lowered slowlyuntil the top disk is exactly a distance of 12 mm from the bottom disk.The shear modulus G′ is measured and recorded by measuring the timerequired for the torsional wave to pass through the sample at platedistances of 12 mm to 6 mm, measured at 1 mm decreasing increments. Theslope of the linear time to disk separation distance plot provides theshear wave velocity used to calculate the shear modulus, G.

In one embodiment, the superabsorbent material useful in making theabsorbent structures of the present invention suitably has a retentioncapacity (CRC) as determined by the Centrifuge Retention Capacity Testdescribed previously of at least 25 grams liquid per gram ofsuperabsorbent material (g/g). In other embodiments, the superabsorbentmaterial may have a retention capacity (CRC) as determined by theCentrifuge Retention Capacity Test of at least about 27.5 g/g and moresuitably at least about 30 g/g.

The superabsorbent material also suitably has a free swell gel bedpermeability (GBP) as determined by the Free Swell Gel Bed PermeabilityTest described previously of at least 350×10⁻⁹ cm², more suitably atleast about 400×10⁻⁹ cm², and still more suitably at least about500×10⁻⁹ cm². In another embodiment, the free swell gel bed permeabilityof the superabsorbent material as determined by the Free Swell Gel BedPermeability Test is suitably at least 575×10⁻⁹ cm², more suitably atleast about 600×10⁻⁹ cm², more suitably at least about 700×10⁻⁹ cm², yetmore suitably at least about 800×10⁻⁹ cm², still more suitably at leastabout 900×10⁻⁹ cm² and even more suitably at least about 1,100×10⁻⁹ cm².

The Absorbency Under Load value at 0.9 psi (also referred to as 0.9 AUL)of the superabsorbent material as determined by the AUL Test describedpreviously is suitably at least 15 grams liquid per gram weight ofsuperabsorbent material (g/g), more suitably at least about 18 g/g,still more suitably at least about 19 g/g and even more suitably atleast about 20 g/g.

EXAMPLES

The following examples are provided to further illustrate the presentinvention and do not limit the scope of the claims. Unless otherwisestated all parts and percentages are by weight.

Example 1

In an insulated, flat-bottomed reaction vessel, 1866.7 g of 50% NaOH wasadded to 3090.26 g of distilled water and cooled to 25° C. 800 g ofacrylic acid was then added to caustic solution and the solution againcooled to 25° C. A second solution of 1600 g of acrylic acid containing9.6 g of polyethylene glycol (300) diacrylate was then added to thefirst solution, followed by cooling to 15° C., the addition of 9.6 g ofmonoallyl ether acrylate with 10 moles of ethoxylation, and additionalcooling to 5° C., all while stirring. The monomer solution was thenpolymerized with a mixture of 100 ppm hydrogen peroxide, 200 ppmazo-bis-(2-amidino-propene)dihydrochloride, 200 ppm sodiumpersulfate and40 ppm ascorbic acid (all aqueous solutions) under adiabatic conditionsand held near T_(max) for 25 minutes. The resulting gel was chopped andextruded with a Hobarth 4M6 commercial extruder, followed by drying in aProcter & Schwartz Model 062 forced air oven at 175° C. for 10 minuteswith upflow and 6 minutes with downflow air on a 20 in×40 in perforatedmetal tray to a final product moisture level of less than 5 wt. %. Thedried material was coarse ground in a Prodeva Model 315-S crusher,milled in an MPI 666-F three stage roller mill and sieved with an MinoxMTS 600DS3V to remove particles greater than 850 microns and smallerthan 150 microns. 400 g of the sieved powder was then blended uniformlywith 0.2 wt. % kaolin (Neogen DGH), followed by the uniform sprayapplication of a solution containing 0.5 wt. % aluminum sulfate, and 1.0wt. % ethylene carbonate in 12 g of water, using a finely atomized spraywhile the SAP particles are fluidized in air. The coated material wasthen heated for 25 minutes at 186° C. in an electrically heated paddledrier.

Example 2

Similar to Example 1 except 12.0 g of polyethylene glycol (300)diacrylate and 12.0 g of monoallyl ether acrylate with 10 moles ofethoxylation were used in the monomer solution.

Example 3

In an insulated, flat-bottomed reaction vessel, 1866.7 g of 50% NaOH wasadded to 3090.26 g of distilled water and cooled to 25° C. 800 g ofacrylic acid was then added to caustic solution and the solution againcooled to 25° C. A second solution of 1600 g of acrylic acid containing120 g of 50 wt. % methoxypolyethyleneglycol (750) monomethacrylate inacrylic acid and 6.0 g of trimethylolpropanetriacrylate with 3 moles ofethoxylation were then added to the first solution, followed by coolingto 15° C., the addition of 10.8 g of allyl ether acrylate with 10 molesof ethoxylation, and additional cooling to 5° C., all while stirring.The monomer solution was then polymerized with a mixture of 100 ppmhydrogen peroxide, 125 ppm azo-bis-(2-amidino-propene)dihydrochloride,300 ppm sodiumpersulfate and 30 ppm sodium erythorbate (all aqueoussolutions) under adiabatic conditions and held near T_(max) for 25minutes. The resulting gel was chopped and extruded with a Hobarth 4M6commercial extruder, followed by drying in a Procter & Schwartz Model062 forced air oven at 175° C. for 10 minutes with upflow and 6 minuteswith downflow air on a 20 in×40 in perforated metal tray to a finalproduct moisture level of less than 5 wt. %. The dried material wascoarse ground in a Prodeva Model 315-S crusher, milled in an MPI 666-Fthree stage roller mill and sieved with an Minox MTS 600DS3V to removeparticles greater than 850 microns and smaller than 150 microns. 400 gof the sieved powder was then blended uniformly with 0.5 wt. % fumedalumina (Degussa Aluminumoxid C), followed by the uniform sprayapplication of a solution containing 0.2 wt. % aluminum sulfate, 0.1 wt.% disodium cocoamphopropionate, 0.5 wt. % tetraethyleneglycol dimethylether, and 1.0 wt. % ethylene carbonate in 5 g of water, using a finelyatomized spray while the SAP particles are fluidized in air. The coatedmaterial was then heated for 20 minutes at 180° C. in a GeneralSignal/BM Model OV-510A-3 forced air oven.

Example 4

In an insulated, flat-bottomed reaction vessel, 1866.7 g of 50% NaOH wasadded to 3090.26 g of distilled water and cooled to 25° C. 800 g ofacrylic acid was then added to caustic solution and the solution againcooled to 25° C. A second solution of 1600 g of acrylic acid containing120 g of 50 wt. % methoxypolyethyleneglycol (750) monomethacrylate inacrylic acid and 14.4 g of trimethylolpropanetriacrylate with 3 moles ofethoxylation were then added to the first solution, followed by coolingto 15° C., the addition of 14.4 g of hydroxymonoallyl ether with 10moles of ethoxylation, and additional cooling to 5° C., all whilestirring. The monomer solution was then polymerized with a mixture of100 ppm hydrogen peroxide, 200 ppmazo-bis-(2-amidino-propene)dihydrochloride, 200 ppm sodiumpersulfate and40 ppm ascorbic acid (all aqueous solutions) under adiabatic conditionsand held near T_(max) for 25 minutes. The resulting gel was chopped andextruded with a Hobarth 4M6 commercial extruder, followed by drying in aProcter & Schwartz Model 062 forced air oven at 175° C. for 10 minuteswith upflow and 6 minutes with downflow air on a 20 in×40 in perforatedmetal tray to a final product moisture level of less than 5 wt. %. Thedried material was coarse ground in a Prodeva Model 315-S crusher,milled in an MPI 666-F three stage roller mill and sieved with an MinoxMTS 600DS3V to remove particles greater than 850 microns and smallerthan 150 microns. 400 g of the sieved powder was then blended uniformlywith 0.5 wt. % fumed silica Aerosil 200 followed by the uniform sprayapplication of a solution containing 0.01 wt. % aluminum sulfate and 1.0wt. % ethylene carbonate in 4 g of water, using a finely atomized spraywhile the SAP particles are fluidized in air. The coated material wasthen heated for 135 minutes at 176° C. in an electrically heated paddledrier.

Experiment 1

Each of the superabsorbent materials set forth in Examples 1-4 weresubjected to the Centrifuge Retention Capacity Test, the Free Swell GelBed Permeability Test, the Shear Modulus Test and the Absorption AgainstPressure Test to determine liquid handling properties thereof. Theresults are set forth in Table 1 below. TABLE 1 Free Swell GBP CRC (g/g)(×10⁻⁹ cm²) G′ (dynes/cm²) AAP (g/g) Example 1 30 612 6899 21.2 Example2 29 862 7777 22.4 Example 3 31 836 5182 19.7 Example 4 27.8 1456 687220.8

For comparison purposes, various commercially available superabsorbentmaterials were also subjected to the Centrifuge Retention Capacity Test,the Free Swell Gel Bed Permeability Test, the Shear Modulus Test and theAbsorption Against Pressure Test to determine liquid handling propertiesthereof. The results are set forth in Table 2 below. Free Swell AAP CRCG′ GBP @0.7 psi (g/g/) (dynes/cm²) (×10⁻⁹ cm²⁾ (g/g) Sanwet 770H 32.44305 58 22.3 Hy-Sorb M 7055 33.1 4276 55 24.2 Hysorb 100 26.3 5649 95 24BASF 2300 33.4 4034 58 19.7 BASF 7050 31.1 5033 62 26.5 BASF 2260 23.99025 553 19.5 BASF ASAP 2000 31.4 3688 50 21 Sumitumo SA60 32.5 3196 3713 Kolon GS3400 30.4 6818 186 22.6 Kolon GS3000 38.9 2811 20 22 DryTech2035M 30.4 7138 35 15.1 DOW S100R 28.2 6032 88 24.3 Aqualic CAB 34.43356 176 17.4 SAP from Pampers 28.4 5746 143 20.6 Baby Dry Diapers SAPfrom Pampers 30.8 5573 130 23.3 Premium diapers SAP from Pampers 28.96866 154 22.2 Cruisers SAP from Luv's 27.3 6954 137 22.0 diapers SAPfrom Huggies 21.5 11490 408 20.9 UltraTrim diaper SAP from Huggies 29.66889 110 10.5 Overnites SAP from Huggies 22.2 11360 325 18.0 SupremesSAP from White 22.1 9785 435 14.4 Cloud diaper SAP from White 22.3 9490373 13.3 Cloud training pants SAP from Walgreens 26.9 7590 278 15.9UltraValue diapers SAP from 22.4 9545 273 14.4 DriBottoms diapers SAPrecovered 39.5 4554 10 13.1 from PaperPak Adult Briefs

Experiment 2

Various superabsorbent materials, set forth in Table 3 below, weresubjected to the Free Swell Gel Bed Permeability Test, the AbsorbencyUnder Load Test and the Centrifuge Retention Capacity Test to determinethe liquid handling properties thereof. Superabsorbent materials A and Bin Table 3 are conventional superabsorbent materials commerciallyavailable from Stockhausen, Inc. of Greensboro, N.C., U.S.A. as modeldesignations SXM 880 and SXM 9543, respectively. Superabsorbent materialC is a superabsorbent material reclaimed from a conventional diaperavailable from Procter & Gamble Co. of Cincinatti, Ohio, U.S.A, underthe tradename Pampers Baby Dry. Superabsorbent materials D-M aresuperabsorbent materials made in accordance with the present inventionby Stockhausen, Inc. of Greensboro, N.C., U.S.A. More particularly,superabsorbent materials D-I are experimental superabsorbent materialsmade by Stockhausen in accordance with the present invention anddesignated SP-1389 (Example 1 set forth above), SP-1390 (Example 2 setforth above), SP-1391, SP-1392, SP-1393 (Example 3 set forth above), andSP 1394 (Example 4 set forth above), respectively. Superabsorbentmaterials J-M are additional experimental superabsorbent materials madeby Stockhausen in accordance with the present invention and designatedSP-1395, SP-1396, SR-1401 and SR-1402, respectively. The results arepresented in Table 3 below. TABLE 3 Super- AUL (g/g) Free Swellabsorbent CRC At at at at GBP Material (g/g) 0.01 psi 0.3 psi 0.6 psi0.9 psi (×10⁻⁹ cm²⁾ A 29.9 46.9 31.8 26.8 23.4 60 B 23.3 35.2 26.2 23.020.6 300 C 29.4 22.8 98 D 31.0 45.6 29.3 24.5 20.1 528 E 28.8 44.4 28.323.8 20.7 846 F 30.5 44.9 28.1 23.0 18.6 467 G 29.0 43.1 26.9 22.7 19.2577 H 30.4 44.5 28.7 22.3 19.0 716 I 28.6 43.4 27.9 22.5 19.5 917 J 30.644.9 29.1 23.5 18.7 624 K 27.5 41.5 27.3 22.1 19.2 1140 L 31.6 46.9 26.220.6 15.6 485 M 27.3 44.2 27.6 23.0 18.5 1173

The absorbent structure of the present invention may be formed in anyconventional manner, such as by being air-formed, air-laid, co-formed,bonded-carded or formed by other known techniques in which fibers andsuperabsorbent material are commingled to form a non-woven web. Forexample, the absorbent structure may alternatively be formed by in-situpolymerization, which typically involves first spraying a monomer ontothe fibers and then polymerizing and crosslinking the monomer to formthe superabsorbent material. As another alternative, the absorbentstructure can be a laminate wherein the superabsorbent material isplaced in a uniform or patterned array on at least one layer ofpermeable and hydrophilic fibers or web or between such layers.

The absorbent structure may be of substantially any shape and sizesuitable for its intended purpose. The absorbent structure may alsocomprise two or more non-woven webs or layers, which may be positionedin side-by-side relationship or surface-to-surface relationship, and allor a portion of adjacent webs or layers may be secured together to formthe absorbent structure.

The superabsorbent material can be substantially homogeneously mixedwith the hydrophilic fibers to provide a uniform distribution of thesuperabsorbent material and fibers throughout the absorbent structure.Alternatively, the superabsorbent material can be distributednon-uniformly within the absorbent structure, such as across the width,along the length and/or through the thickness of the structure to definediscrete target regions or zones of the structure within which thesuperabsorbent material is distributed. The concentration ofsuperabsorbent material within the absorbent structure can also benon-uniform through all or part of the thickness, across all or part ofthe width and/or along all or part of the length of the absorbentstructure.

In general, the overall concentration of superabsorbent material withinthe absorbent structure is suitably about 90 weight percent or lessbased on the total weight of the absorbent structure, but is in anyevent greater than zero. In one embodiment, the concentration ofsuperabsorbent material within the absorbent structure is suitably inthe range of about 30 to about 90 weight percent, more suitably in therange of about 40 to about 90 weight percent and even more suitably inthe range of about 40 to about 80 weight percent. In another embodimentthe concentration of superabsorbent material within the absorbentstructure is in the range of about 40 to about 60 weight percent.

The absorbent structure may or may not be wrapped or otherwiseencompassed by a suitable tissue or web wrap for maintaining theintegrity and/or shape of the absorbent structure.

Experiment 3

Six different absorbent structures (coded as structures 1-6 in Table 2below) were made in a laboratory air-forming apparatus and subjected tovarious tests including a Saturation Capacity Test, an AbsorbentStructure Permeability Test, a Fluid Intake Flowback Evaluation (FIFE)Test and a Vertical Wicking Test, all of which are described laterherein, to assess the liquid handling properties of the absorbentstructures. Each of the absorbent structures comprised a non-woven webof hydrophilic fibers (and more particularly hydrophilic fibersavailable from Weyerhauser of Federal Way, Wash., U.S.A. as modeldesignation NB-416) and one of six different superabsorbent materials.

The first five absorbent structure codes reflect construction of theabsorbent structure using superabsorbent materials described previouslyas being useful in making absorbent structures according to the presentinvention. More particularly, Code 1 corresponds to an absorbentstructure incorporating superabsorbent material “E” from Table 3, Code 2corresponds to an absorbent structure incorporating superabsorbentmaterial “G” from Table 3, Code 3 corresponds to an absorbent structureincorporating superabsorbent material “I” from Table 3, Code 4corresponds to an absorbent structure incorporating superabsorbentmaterial “K” from Table 3 and Code 5 corresponds to an absorbentstructure incorporating superabsorbent material “M” from Table 3. Code 6corresponds to an absorbent structure incorporating the conventionalsuperabsorbent material identified as superabsorbent material “B” inTable 3 and available from Stockhausen, Inc. as model designation SXM9543.

For each absorbent structure code, three different type samples wereproduced for testing, designated in Table 4 below by the letters a, band c. For “a” type samples the target superabsorbent concentrationwithin the absorbent structure sample was about 45 percent and thetarget density was about 0.222 g/cm³; for “b” type samples the targetsuperabsorbent concentration was about 45 percent and the target densitywas about 0.353 g/cm³; and for “c” type samples the targetsuperabsorbent concentration was about 65 percent and the target densitywas about 0.353 g/cm³. The target basis weight of each absorbentstructure sample was approximately 600 grams per square meter (gsm).

The Saturation Capacity Test, Absorbent Structure Permeability Test,Fluid Intake Flowback Evaluation (FIFE) Test and Vertical Wicking Testwere performed for each of the absorbent structure samples and theresults are recorded in Table 4. TABLE 4 Absorbent Structure CompositionSAM Absorbent Structure Vertical type Sat. Permeability Intake Rate(g/sec) @ 75 ml Load Wicking (from SAM Density Cap. Free at 0.3 1st 2nd3rd 4th Capacity Code Table 3) (%) g/cm³ g/g Swell ×10⁻⁸ cm² psi ×10⁻⁸cm² Insult Insult Insult Insult g 1a E 45.0 0.222 18.0 24.2 17.6 1.811.01 2.81 2.48 103.7 2a G 45.0 0.222 17.2 32.7 23.5 2.05 1.06 2.82 2.30102.4 3a I 45.0 0.222 18.0 39.4 26.3 2.12 1.01 2.64 2.34 106.5 4a K 45.00.222 18.4 32.7 21.9 2.11 1.02 2.84 2.53 105.8 5a M 45.0 0.222 19.1 25.912.4 1.97 1.02 2.40 2.05 114.3 6a B 45.0 0.222 15.6 25.0 17.1 1.87 1.021.68 1.47 93.4 1b E 45.0 0.353 17.2 23.1 11.6 1.42 1.02 2.58 2.29 120.82b G 45.0 0.353 17.5 25.3 17.9 1.56 1.03 2.10 1.76 123.9 3b I 45.0 0.35318.1 41.0 16.9 1.44 1.02 2.27 1.96 107.7 4b K 45.0 0.353 17.3 29.3 23.81.62 1.01 2.47 2.07 108.0 5b M 45.0 0.353 18.3 14.8 15.4 1.51 1.02 1.741.53 121.6 6b B 45.0 0.353 14.9 21.9 9.4 1.28 1.01 1.17 0.95 96.7 1c E65.0 0.353 22.7 22.3 8.9 1.35 1.01 1.79 1.64 110.8 2c G 65.0 0.353 22.018.2 8.3 1.35 1.01 1.88 1.74 122.0 3c I 65.0 0.353 21.7 22.8 9.7 1.461.01 2.27 2.00 118.1 4c K 65.0 0.353 21.5 24.2 9.6 1.39 1.01 1.78 1.53121.8 5c M 65.0 0.353 22.8 14.4 5.7 1.29 1.01 1.43 1.34 118.9 6c B 65.00.353 17.3 14.6 8.9 1.18 1.02 1.53 1.43 94.8

In general, the absorbent structures made in accordance with the presentinvention (e.g., codes 1-5) exhibited enhanced liquid handlingcharacteristics relative to the absorbent structure (code 6)incorporating the conventional superabsorbent material. For example,with respect to the intake rate as determined by the FIFE test, theabsorbent structures made in accordance with the present inventiongenerally exhibited greater intake rates than the absorbent structureincorporating the conventional superabsorbent material, particularlyupon repeated insults of the absorbent structure (e.g., upon third andfourth insults thereof). The saturation capacity, absorbent structurepermeability (both free swell and under 0.3 psi load) and wickingcapacity of the absorbent structures made in accordance with the presentinvention were also generally better than those exhibited by theabsorbent structure incorporating the conventional superabsorbentmaterial.

Fluid Intake Flowback Evaluation Test

The Fluid Intake Flowback Evaluation (FIFE) Test determines the amountof time required for an absorbent structure, and more particularly asample thereof, to take in (but not necessarily absorb) a known amountof test solution (0.9 weight percent solution of sodium chloride indistilled water at room temperature). A suitable apparatus forperforming the FIFE Test is shown in FIGS. 5 and 6 and is generallyindicated at 200. The test apparatus 200 comprises upper and lowerassemblies, generally indicated at 202 and 204 respectively, wherein thelower assembly comprises a generally 14 inch (35.56 cm) by 8 inch (20.32cm) rectangular plate 206 constructed of a transparent material such asPlexiglas and a generally 6 inch (15.24 cm) by 3 inch (7.62 cm)rectangular platform 207 centered on the plate for centering theabsorbent structure sample during the test.

The upper assembly 202 comprises a generally rectangular plate 208constructed similar to the lower plate 206 and having a central opening210 formed therein. A cylinder 212 having an inner diameter of about 2inches (about 5.08 cm) and a height of about 4 inches (about 10.16 cm)is secured to the upper plate 208 at the central opening 210 and extendsupward substantially perpendicular to the upper plate. The centralopening 210 of the upper plate 208 should have a diameter at least equalto the inner diameter of the cylinder 212 where the cylinder is mountedon top of the upper plate. However, the diameter of the central opening210 may instead be sized large enough to receive the outer diameter ofthe cylinder 212 within the opening so that the cylinder is secured tothe upper plate 208 within the central opening.

Pin elements 214 are located near outside corners of the lower plate206, and corresponding recesses 216 in the upper plate 208 are sized toreceive the pin elements to properly align and position the upperassembly 202 on the lower assembly during testing. The weight of theupper assembly 202 (e.g., the upper plate 208 and cylinder 212) isapproximately 845 grams.

To run the FIFE Test, an absorbent structure sample 218 (either formedto the desire size or removed from an absorbent article and cut to thedesired size) having length and width dimensions of about 14 inches(35.56 cm) by about 3 inches (7.68 cm) is weighed, with the tissue wrapon, and the weight is recorded in grams. The sample 218 is then centeredon the platform 207 of the lower assembly. The upper assembly 202 isplaced over the sample in opposed relationship with the lower assembly,with the pins 214 of the lower plate seated in the recesses 216 formedin the upper plate 208 and the cylinder 212 generally centered over thesample. Approximately 75 mL of the test solution (referred to herein asa first insult) is poured into the top of the cylinder 212 (e.g.,generally at the height of the top of the cylinder) and allowed to flowdown into the absorbent structure sample 218. A stopwatch is startedwhen the first drop of solution contacts the sample 218 and is stoppedwhen the liquid ring between the edge of the cylinder 212 and the sampledisappears. The reading on the stopwatch is recorded to two decimalplaces and represents the intake time (in seconds) required for thefirst insult to be taken into the absorbent structure sample 218.

A time period of fifteen minutes is allowed to elapse, after which asecond insult equal to the first insult is poured into the top of thecylinder 212 and again the intake time is measured as described above.The procedure is repeated for a third insult and then a fourth insult aswell. An intake rate (in milliliters/second) for each of the fourinsults is determined by dividing the amount of solution (e.g., 75 mL)used for each insult by the intake time measured for the correspondinginsult.

At least 3 samples of each absorbent structure are subjected to the FIFETest and the results are averaged to determine the intake time andintake rate of the absorbent structure.

Absorbent Structure Permeability Test

The Absorbent Structure Permeability Test is used to determine thepermeability of the absorbent structure, and more particularly a“z-direction” permeability of the absorbent structure based on liquidflow through the thickness of the structure. This test is substantiallysimilar to the Free Swell Gel Bed Permeability Test set forth above,with the following noted exceptions. Referring back to FIGS. 1 and 2,instead of the cylinder height being about 5 cm, the cylinder heightshould be about 10 cm. Also, instead of particulate superabsorbentmaterial being placed in the sample container, a circular absorbentstructure sample 68 (e.g., either formed or otherwise cut from a largerabsorbent structure), with any tissue wrap removed and having across-sectional diameter of about 6 cm is placed in the sample container30 at the bottom of the cylinder 34 in contact with the screen 64. Thesample container (without the piston and weight therein) is thensubmerged in a 0.9 weight percent saline solution for a time period ofabout 60 minutes to saturate the absorbent structure. The same heightmeasurements obtained for the Free Swell Gel Bed Permeability Test aretaken, e.g., with the container 30 empty and with the absorbentstructure sample within the container and saturated.

The absorbent structure permeability measurement is initiated bydelivering a continuous flow of saline solution into the samplecontainer 30 with the saturated absorbent structure, the piston 36, andthe weight 48 inside. The saline solution is delivered to the container30 at a flow rate sufficient to maintain a fluid height of about 7.8 cm(instead of the 4 cm used for the Free Swell Gel Bed Permeability Test)above the bottom of the sample container. The quantity of fluid passingthrough the absorbent structure versus time is measured gravimetrically.Data points are collected every second for at least twenty seconds oncethe fluid level has been stabilized to and maintained at about 7.8 cm inheight. The flow rate Q through the absorbent structure sample 68 isdetermined in units of grams/second (g/s) by a linear least-square fitof fluid passing through the container (in grams) versus time (inseconds). The permeability of the absorbent structure is then determinedusing the equation set forth above for the Free Swell Gel BedPermeability Test.

Where the Absorbent Structure Permeability Test is conducted asdescribed above, and more particularly where the absorbent structuresample is submerged in the solution without the piston and weightthereon, the test is said to be conducted under “free swell” conditionswhereby the absorbent structure is allowed to swell free of anyrestraining load. In a variation of this test, the piston and weight maybe placed on the sample within the container and then the entireassembly can be submerged so that a load (e.g., approximately 0.3 psi)is applied to the sample as the sample becomes saturated and swells.When conducted in this manner the test is referred to as being conducted“under load.”

Vertical Wicking Test

The Vertical Wicking Test determines the amount of test solution (0.9weight percent solution of sodium chloride in distilled water) that willwick upward into an absorbent structure during a 30 minute period.

A sample of the absorbent structure to be tested is prepared to havedimensions of about 3 inches wide by about 7 inches long, e.g., eitherformed or otherwise cut from a larger absorbent structure. The sample isthen clamped to one face of an acrylic board measuring 25 cm high by 15cm wide by 0.5 cm thick such that one end of the sample extends slightlybeyond the bottom end of the acrylic board. The sample is further heldin place on the board by two clamps extended around the side edges ofthe board so as to grasp the side edges of the sample near the top ofthe sample. The side of the board may be scaled in 1 mm increments tomeasure the vertical height of the wicked solution.

The sample (and board) is then hung from a free swinging support clampand the sample is lowered into a reservoir of the test solution untilthe lower end of the sample contacts the solution. A timer with onesecond increments is started just as the sample contacts the liquid. Thesolution is allowed to be taken into the sample and wick upward thereinfor a period of about thirty minutes. The sample is then removed fromthe reservoir and taken off of the board and weighed. The differencebetween the weight of the sample after thirty minutes and the dry weightof the sample is the wicking capacity, in grams weight (g) of theabsorbent structure.

Liquid Saturation Capacity Test

The following test is used to determine a saturation capacity of anabsorbent structure. With reference to FIG. 7, an absorbent structuresample 308 having length and width dimensions of approximately fourinches by four inches (approximately 10.16 cm by 10.16 cm) is weighedwith any tissue wrap material on and the weight in grams is recorded.The sample 308 is then wrapped in paper toweling (not shown), such asScott paper towel available from Kimberly-Clark Worldwide Inc. ofNeenah, Wis., U.S.A., and submerged in an excess quantity of 0.9 weightpercent sodium chloride solution in distilled water at room temperature(e.g., about 23 degrees Celsius) for about twenty minutes. After thistime period, the sample 308 is removed from the test solution and placedon a test apparatus, indicated generally at 300 in FIG. 7, comprising avacuum box 302, a TEFLON fiberglass screen 304 having 0.25 inch (0.6 cm)openings and supported by the vacuum box, and a flexible rubber cover306 sized for overlaying the screen on the vacuum box.

More particularly, the absorbent structure sample 308 (with toweling) isplaced uncovered (by the rubber cover) on the screen 304 and allowed todrip dry for about one minute. The rubber cover 306 is then placed overthe sample 308 and screen 304 (e.g., to generally form a seal over thevacuum box 302) and a vacuum (V) of about 0.5 pounds/square inch (about34.5 dynes/square cm) is drawn on the vacuum box (and hence the sample)for a period of about five minutes. The sample 308 is then removed fromthe apparatus and the toweling is taken off the sample, making an effortto recover loose fibers and superabsorbent particles along with thesample. The recovered sample is again weighed and the weight in grams isrecorded. The saturation capacity of the sample is determined bysubtracting the dry weight of the sample from the weight of therecovered sample after application of the vacuum and then dividing bythe dry weight of the sample and is recorded as grams of liquid retainedper gram of absorbent structure (g/g).

If absorbent structure fibers and/or superabsorbent material are drawnthrough the fiberglass screen into the vacuum box during testing, ascreen having smaller openings should be used and the test should bere-done. Alternatively, a piece of tea bag material or other similarmaterial can be placed between the sample and the screen and the totalretention capacity adjusted for the liquid retained by the tea bag orother material.

At least three samples of each absorbent structure are tested and theresults are averaged to provide the retention capacity (e.g., total andnormalized retention capacity) of the absorbent structure.

As described previously, the absorbent structure formed in accordancewith the present invention may be incorporated in an absorbent article.As used herein, an absorbent article refers to an article which may beplaced against or in proximity to the body of the wearer (e.g.,contiguous to the body) to absorb and/or retain various waste dischargedfrom the body. Some absorbent articles, such as disposable articles, areintended to be discarded after a limited period of use instead of beinglaundered or otherwise restored for reuse. In one embodiment, anabsorbent article of the present invention comprises an outer cover, abodyside liner positioned in facing relation with the outer cover andadapted for contiguous relationship with the body of the wearer, and anabsorbent body disposed between the outer cover and the liner. Thebodyside liner may be generally coextensive with the outer cover, or mayinstead overlie an area which is larger or smaller than the area of theouter cover, as desired.

In one embodiment, the outer cover is stretchable and may or may not besomewhat elastic. More particularly, the outer cover is sufficientlyextensible such that once stretched under the weight of the insultedabsorbent body, the outer cover will not retract substantially backtoward its original position. However, it is contemplated that the outercover may instead be generally non-extensible and remain within thescope of this invention.

The outer cover may be a single layer structure or it may be amulti-layered laminate structure to provide desired levels ofextensibility as well as liquid impermeability and vapor permeability.For example, the outer cover can be a two-layer construction, includingan outer layer constructed of a vapor permeable material and an innerlayer constructed of a liquid impermeable material, with the two layersbeing secured together by a suitable laminate adhesive. The vaporpermeable outer layer can be any suitable material and is desirably onewhich provides a generally cloth-like texture. Suitable materials forthe outer layer include non-woven webs, woven materials and knittedmaterials. Non-woven fabrics or webs have been formed from many knownprocesses, for example, bonded carded web processes, meltblowingprocesses and spunbonding processes.

The liquid impermeable inner layer of the outer cover can be eithervapor permeable (i.e., “breathable”) or vapor impermeable. The innerlayer is desirably manufactured from a thin plastic film, although otherflexible liquid impermeable materials may also be used. Moreparticularly, the inner layer can be made from either cast or blown filmequipment, can be coextruded and can be embossed if so desired. It isunderstood that the inner layer may otherwise be made from any suitablenon-elastic polymer composition and may include multiple layers. Wherethe inner layer is vapor permeable, it may contain such fillers asmicropore developing fillers, e.g. calcium carbonate; opacifying agents,e.g. titanium dioxide; and antiblock additives, e.g. diatomaceous earth.Suitable polymers for the inner layer include but are not limited tonon-elastic extrudable polymers such as polyolefin or a blend ofpolyolefins, nylon, polyester and ethylene vinyl alcohol. Moreparticularly, useful polyolefins include polypropylene and polyethylene.Other useful polymers include those described in U.S. Pat. No. 4,777,073to Sheth, assigned to Exxon Chemical Patents Inc., such as a copolymerof polypropylene and low density polyethylene or linear low densitypolyethylene.

The bodyside liner is suitably pliable, soft feeling, and nonirritatingto the wearer's skin, and is employed to help isolate the wearer's skinfrom the absorbent body. The liner is desirably less hydrophilic thanthe absorbent body to present a relatively dry surface to the wearer,and is sufficiently porous to be liquid permeable to thereby permitliquid to readily penetrate through its thickness. A suitable bodysideliner may be manufactured from a wide selection of web materials.Various woven and nonwoven fabrics including either or both syntheticand natural fibers can be used for the liner. For example, the bodysideliner may be composed of a meltblown or spunbonded web of the desiredfibers, and may also be a bonded-carded-web. Layers of differentmaterials that may have different fiber deniers can also be used. Thevarious fabrics can be composed of natural fibers, synthetic fibers orcombinations thereof.

The various components of the absorbent article are assembled togetherusing a suitable form of attachment, such as adhesive, sonic bonds,thermal bonds or combinations thereof. For example, in one embodimentthe outer cover and absorbent body are secured to each other with linesof adhesive, such as a hot melt or pressure-sensitive adhesive. Thebodyside liner is also secured to the outer cover and may also besecured to the absorbent body using the same forms of attachment.

In accordance with the present invention, the absorbent body comprisesat least in part an absorbent structure as described previously herein.It is contemplated that the absorbent body may comprise one or more ofthe absorbent structures, such as in overlaid or side-by-siderelationship, and/or it may comprise one or more layers in addition tothe absorbent structure, such as a surge layer, without departing fromthe scope of this invention.

It will be appreciated that details of the foregoing embodiments, givenfor purposes of illustration, are not to be construed as limiting thescope of this invention. Although only a few exemplary embodiments ofthis invention have been described in detail above, those skilled in theart will readily appreciate that many modifications are possible in theexemplary embodiments without materially departing from the novelteachings and advantages of this invention. For example, featuresdescribed in relation to one embodiment may be incorporated into anyother embodiment of the invention.

Accordingly, all such modifications are intended to be included withinthe scope of this invention, which is defined in the following claimsand all equivalents thereto. Further, it is recognized that manyembodiments may be conceived that do not achieve all of the advantagesof some embodiments, particularly of the preferred embodiments, yet theabsence of a particular advantage shall not be construed to necessarilymean that such an embodiment is outside the scope of the presentinvention.

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description shall be interpreted asillustrative and not in a limiting sense.

1. An absorbent structure comprising at least in part a superabsorbentmaterial having a retention capacity (CRC) as determined by a CentrifugeRetention Capacity Test of at least about 25 g/g and a free swell gelbed permeability (GBP) as determined by a Free Swell Gel BedPermeability Test of at least 575×10⁻⁹ cm².
 2. An absorbent structure asset forth in claim 1 wherein the superabsorbent material has a freeswell gel bed permeability as determined by the Free Swell Gel BedPermeability Test of at least about 600×10⁻⁹ cm².
 3. An absorbentstructure as set forth in claim 2 wherein the superabsorbent materialhas a free swell gel bed permeability as determined by the Free SwellGel Bed Permeability Test of at least about 800×10⁻⁹ cm².
 4. Anabsorbent structure as set forth in claim 3 wherein the superabsorbentmaterial has a free swell gel bed permeability as determined by the FreeSwell Gel Bed Permeability Test of at least about 1,100×10⁻⁹ cm².
 5. Anabsorbent structure as set forth in claim 1 wherein the superabsorbentmaterial has a retention capacity as determined by the CentrifugeRetention Capacity Test of at least about 27.5 g/g.
 6. An absorbentstructure as set forth in claim 5 wherein the superabsorbent materialhas a free swell gel bed permeability as determined by the Free SwellGel Bed Permeability Test of at least about 600×10⁻⁹ cm².
 7. Anabsorbent structure as set forth in claim 6 wherein the superabsorbentmaterial has a free swell gel bed permeability as determined by the FreeSwell Gel Bed Permeability Test of at least about 800×10⁻⁹ cm².
 8. Anabsorbent structure as set forth in claim 1 wherein the superabsorbentmaterial has a retention capacity as determined by the CentrifugeRetention Capacity Test of at least about 30 g/g.
 9. An absorbentstructure as set forth in claim 8 wherein the superabsorbent materialhas a free swell gel bed permeability as determined by the Free SwellGel Bed Permeability Test of at least about 600×10⁻⁹ cm².
 10. Anabsorbent structure as set forth in claim 9 wherein the superabsorbentmaterial has a free swell gel bed permeability as determined by the FreeSwell Gel Bed Permeability Test of at least about 700×10⁻⁹ cm².
 11. Anabsorbent structure as set forth in claim 1 wherein the superabsorbentmaterial has an Absorbency Under Load (AUL) at 0.9 psi as determined byan Absorbency Under Load Test of at least about 15 g/g.
 12. An absorbentstructure as set forth in claim 11 wherein the superabsorbent materialhas an Absorbency Under Load (AUL) at 0.9 psi as determined by theAbsorbency Under Load Test of at least about 17.5 g/g.
 13. An absorbentstructure as set forth in claim 12 wherein the superabsorbent materialhas an Absorbency Under Load (AUL) at 0.9 psi as determined by theAbsorbency Under Load Test of at least about 19 g/g.
 14. An absorbentstructure as set forth in claim 13 wherein the superabsorbent materialhas an Absorbency Under Load (AUL) at 0.9 psi as determined by theAbsorbency Under Load Test of at least about 20 g/g.
 15. An absorbentstructure as set forth in claim 1 further comprising at least one ofhydrophilic fibers and hydrophobic fibers.
 16. An absorbent structure asset forth in claim 15 wherein the hydrophilic fibers comprise cellulosicfibers.
 17. An absorbent structure as set forth in claim 1 where thesuperabsorbent material in the absorbent structure comprises in therange of about thirty percent to about ninety percent of the weight ofthe absorbent structure.
 18. An absorbent structure as set forth inclaim 1 wherein the superabsorbent material comprises one of at leastabout 75 weight percent anionic polymer and at least about 75 weightpercent cationic polymer.
 19. An absorbent structure as set forth inclaim 18 wherein the superabsorbent material comprises one of at leastabout 85 weight percent anionic polymer and at least about 85 weightpercent cationic polymer.
 20. An absorbent structure as set forth inclaim 19 wherein the superabsorbent material comprises one of at leastabout 90 weight percent anionic polymer and at least about 90 weightpercent cationic polymer.
 21. An absorbent article comprising at leastin part the absorbent structure of claim
 1. 22. An absorbent article asset forth in claim 21 wherein the absorbent article is one of a diaper,a training pant, a feminine hygiene product and an incontinence product.23. An absorbent structure comprising at least in part a superabsorbentmaterial having a retention capacity (CRC) as determined by a CentrifugeRetention Capacity Test of at least about 25 g/g, an Absorbency UnderLoad (AUL) at 0.9 psi as determined by an Absorbency Under Load Test ofat least 18 and a free swell gel bed permeability (GBP) as determined bya Free Swell Gel Bed Permeability Test of at least about 350×10⁻⁹ cm².24. An absorbent structure as set forth in claim 23 wherein thesuperabsorbent material has a free swell gel bed permeability asdetermined by the Free Swell Gel Bed Permeability Test of at least about400×10⁻⁹ cm².
 25. An absorbent structure as set forth in claim 24wherein the superabsorbent material has a free swell gel bedpermeability as determined by the Free Swell Gel Bed Permeability Testof at least about 600×10⁻⁹ cm².
 26. An absorbent structure as set forthin claim 25 wherein the superabsorbent material has a free swell gel bedpermeability as determined by the Free Swell Gel Bed Permeability Testof at least about 800×10⁻⁹ cm².
 27. An absorbent structure as set forthin claim 26 wherein the superabsorbent material has a free swell gel bedpermeability as determined by the Free Swell Gel Bed Permeability Testof at least about 1,100×10⁻⁹ cm².
 28. An absorbent structure as setforth in claim 23 wherein the superabsorbent material has a retentioncapacity as determined by the Centrifuge Retention Capacity Test of atleast about 27.5 g/g.
 29. An absorbent structure as set forth in claim28 wherein the superabsorbent material has a free swell gel bedpermeability as determined by the Free Swell Gel Bed Permeability Testof at least about 400×10⁻⁹ cm².
 30. An absorbent structure as set forthin claim 29 wherein the superabsorbent material has a free swell gel bedpermeability as determined by the Free Swell Gel Bed Permeability Testof at least about 600×10⁻⁹ cm².
 31. An absorbent structure as set forthin claim 30 wherein the superabsorbent material has a free swell gel bedpermeability as determined by the Free Swell Gel Bed Permeability Testof at least about 800×10⁻⁹ cm².
 32. An absorbent structure as set forthin claim 28 wherein the superabsorbent material has a retention capacityas determined by the Centrifuge Retention Capacity Test of at leastabout 30 g/g.
 33. An absorbent structure as set forth in claim 32wherein the superabsorbent material has a free swell gel bedpermeability as determined by the Free Swell Gel Bed Permeability Testof at least about 400×10⁻⁹ cm².
 34. An absorbent structure as set forthin claim 33 wherein the superabsorbent material has a free swell gel bedpermeability as determined by the Free Swell Gel Bed Permeability Testof at least about 600×10⁻⁹ cm².
 35. An absorbent structure as set forthin claim 34 wherein the superabsorbent material has a free swell gel bedpermeability as determined by the Free Swell Gel Bed Permeability Testof at least about 700×10⁻⁹ cm².
 36. An absorbent structure as set forthin claim 23 wherein the superabsorbent material has an Absorbency UnderLoad (AUL) at 0.9 psi as determined by the Absorbency Under Load Test ofat least about 19 g/g.
 37. An absorbent structure as set forth in claim36 wherein the superabsorbent material has an Absorbency Under Load(AUL) at 0.9 psi as determined by the Absorbency Under Load Test of atleast about 20 g/g.
 38. An absorbent structure as set forth in claim 23further comprising at least one of hydrophilic fibers and hydrophobicfibers.
 39. An absorbent structure as set forth in claim 38 wherein thehydrophilic fibers comprise cellulosic fibers.
 40. An absorbentstructure as set forth in claim 23 where the superabsorbent material inthe absorbent structure comprises in the range of about thirty percentto about ninety percent of the weight of the absorbent structure.
 41. Anabsorbent structure as set forth in claim 23 wherein the superabsorbentmaterial comprises one of at least about 75 weight percent anionicpolymer and at least about 75 weight percent cationic polymer.
 42. Anabsorbent structure as set forth in claim 41 wherein the superabsorbentmaterial comprises one of at least about 85 weight percent anionicpolymer and at least about 85 weight percent cationic polymer.
 43. Anabsorbent structure as set forth in claim 42 wherein the superabsorbentmaterial comprises one of at least about 90 weight percent anionicpolymer and at least about 90 weight percent cationic polymer.
 44. Anabsorbent article comprising at least in part the absorbent structure ofclaim
 23. 45. An absorbent article as set forth in claim 44 wherein theabsorbent article is one of a diaper, a training pant, a femininehygiene product and an incontinence product.